Audio driver circuits are used in many of today's electronic devices (e.g. cell phones, portable music players, etc.) to drive sound producing devices (SPD), such as speakers. Typically, an audio driver circuit comprises a digital-to-analog converter (DAC) and an amplifier. The DAC receives digital signals (representing, for example, music, speech, or other sounds) and converts them into analog signals. The analog signals are then amplified by the amplifier and provided to the SPD to cause the SPD to produce sounds that correspond to the digital signals.
A phenomenon that has been observed with current audio driver circuits is that during the power up and/or power down sequences, a “pop” noise is often produced by the SPD. This pop noise is caused by the rapid change in voltage at the output of the audio driver circuit's amplifier. Because this pop noise is often annoying to users and can sometimes cause damage to sensitive components, eliminating this noise is an objective of many audio driver circuits.
One technique that has been used to eliminate the pop noise is to employ a large capacitor between the output of the DAC and the input of the amplifier. This capacitor has the effect of introducing a long time constant, which slows down the ramp up and ramp down of the common mode voltage at the output of the amplifier, thereby eliminating or at least reducing the pop noise.
While this approach is effective for reducing pop noise, it has a number of practical drawbacks. The first drawback is increased cost. This approach requires the use of a relatively large capacitor. Such large capacitors cannot be fabricated on a semiconductor chip. Thus, the capacitor cannot be incorporated onto the chip on which the audio driver circuit is fabricated, which means that the capacitor has to be implemented as an external (i.e. off-chip) discrete capacitor. The use of external discrete components significantly increases cost. Another drawback, which flows from the fact that the capacitor has to be implemented off-chip, is that the implementation requires more board space. In small electronic devices, board space is very scarce, and many devices may not have enough board space to accommodate the capacitor. Thus, this approach may not even be implementable. Yet another drawback, which also flows from the fact that the capacitor has to be implemented off-chip, is that the chip on which the audio driver circuit is fabricated has to have an additional pin for accommodating the off-chip capacitor. In many applications, pin count is constrained and adding a pin just for the off-chip capacitor is not an option. Thus, this approach may again not be implementable.
In view of the above drawbacks, an improved approach for eliminating/reducing pop noise is needed.